Collect samples in advance and conduct these experiments at a public demonstration station.

OVERVIEW

The common sources of carbon dioxide that contribute to the greenhouse effect are brought home to your students through this popular activity in which they compare the carbon dioxide content in their breath and in car exhaust.

OBJECTIVES

Students will:

Improve their abilities to apply a chemical testing technique in conducting an experiment.

Practice sampling and testing procedures like those used in scientific studies of the atmosphere.

Draw out the distinction between “natural” and “industrial” sources of carbon dioxide in the atmosphere.

Communicate the relative contributions of the industrialized and developing nations to the global warming problem.

Standard B: Physical Science—Properties and changes of properties in matter

Standard F: Science in Personal and Social Perspectives—Population, resources, and environments

BACKGROUND

Carbon dioxide is a natural component of Earth’s atmosphere and plays an important role in maintaining a livable temperature on the planet. All animals, including humans, produce carbon dioxide when they breathe. Carbon is also released from rocks through natural weathering processes and is released in the exchange of gases between the oceans and the atmosphere. Other processes such as photosynthesis (short-term) and sedimentation (long-term) absorb carbon dioxide. Through this natural cycle, Earth maintains a carbon balance.

Human actions, such as the burning of fossil fuels, are adding carbon dioxide to the atmosphere at a rate faster than natural processes can absorb. The long-term processes that remove carbon dioxide from the atmosphere (ocean uptake and sedimentation) work very slowly.

As a result, carbon dioxide levels in the atmosphere are increasing. Higher concentrations of carbon dioxide hold more infrared energy in the atmosphere than occurs naturally. This added heat warms the atmosphere and Earth’s surface. The vast majority of the scientific community agrees that increasing carbon dioxide in the atmosphere leads to gradual warming of Earth’s climate.

In this activity, students use bromothymol blue (BTB) to test gas samples for carbon dioxide. They apply the technique in an experiment to investigate sources of carbon dioxide in their immediate environment. From these tests, students learn that the concentration of carbon dioxide exhaled by humans is less than in car exhaust. They discuss the implications of their results for controlling the amounts of carbon dioxide released into the atmosphere. Then, through a follow-up assignment, they learn how much the various nations of the world contribute to the increase of carbon dioxide in the atmosphere.

(NOTE: Obtaining samples of gas from auto exhausts is much easier than it might seem, and it is exciting for students to see the samples collected in front of their eyes. Observing how rapidly the balloons inflate is also a graphic demonstration of how much gas cars release into the atmosphere every second. If you are concerned about safety issues relating to obtaining samples, use adult volunteers to assist.)

BEFORE YOU BEGIN

On the Day Before the Activity:

1. Prepare the BTB solution. Make a one gallon solution of BTB and water. If beginning with the concentrated liquid, fill a one gallon (3.8 liter) bottle 9/10ths full with tap water and add BTB concentrate until the solution is a deep, transparent blue. The exact concentration is not critical. However, you should test the solution by pouring ½ oz. (15 ml) solution into one of the small clear cups. Using a straw, bubble one lungful of breath through the small cup of solution. If the solution turns green, it is OK. If it stays blue, or only slightly bluish green, it is too concentrated. Pour out some solution, add more water, and test again. If it turns yellow, it is too diluted, and you need to add more BTB.

If you start with BTB in powdered form, the instructions are the same, except you must first prepare the concentrate. Put 0.03 oz. (1 gram) of BTB powder into a 2.1 pint (1 liter) container. Add 1.1 tablespoons (16 ml) of 0.1 molar sodium hydroxide (0.1 M NaOH) and dissolve the crystals of BTB. Add 2.1 pints (1 liter) of water to make 2.1 pints (1 liter) of concentrate. For each group of four students, fill a squirt bottle with about 6 oz. (0.2 liter) of prepared BTB solution.

2. Use a car with a round exhaust pipe (square tailpipes are difficult to seal). Prepare a cone for collecting car exhaust by rolling up a manila folder the long way. One end must be larger than the opening of a car’s tailpipe, and the other end must be small enough for a balloon to fit over it. Use plenty of tape to hold the cone in shape and to make the sides of the cone airtight. Trim the ends of the cone with scissors if necessary. Make a spare cone and have tape, folders, and scissors on hand when you collect the gas. Practice filling a balloon with car exhaust before class. Approach the exhaust pipe from the side and hold your breath when filling the balloons so you do not inhale the gases.

3. Make one copy of the two-sheet assignment:
Carbon Dioxide In the
Atmosphere: Who Contributes and How
Much?
and the
Four Gas Samples: Data
Sheet
for each student. Make one copy of the
Four Gas Samples: Observation
Sheet
for each group of four students.

On the Day of the Activity:

1. Park the car within close walking distance. Preferably, the class should not have to cross a street to get to the car. Decide where the students should stand to watch you collect the gas samples so they can see what is happening, but are not in traffic. Since auto exhaust contains toxic gases, the students should stand back far enough so they do not have to breathe in car exhaust.

2. Assemble the equipment that each student team will need, using trays for easy distribution. Use the amounts of BTB solution indicated above in this section.

3. Have the air pump available to be passed around from group to group.

WHAT TO DO

Introduction:

1. Introduce the concept of global warming and discuss why we might be concerned about levels of carbon dioxide in the atmosphere.

2. Explain that BTB is an indicator that can be used to test for the presence and relative concentration of carbon dioxide. On the chalkboard or on the butcher paper, draw the scale of how BTB changes color when exposed to different amounts of carbon dioxide.

3. Explain to the class that among the sources of carbon dioxide in the atmosphere are car exhaust and the breath of humans and animals. The students will compare the concentration of carbon dioxide from a car’s exhaust with other samples.

4. Tell the students they will take a brief field trip to a car to collect samples of exhaust, which they will analyze using the BTB method.

Collecting Samples of Car Exhaust:

1. Organize the class into groups, then issue each group one twist-tie and one balloon. (Bring along several extra balloons and twist-ties, in case you have breakage. All balloons for the car exhaust should be the same color. Also bring the funnel, as well as an extra funnel, folder, scissors, and tape).

2. Tell the students to stay in groups to follow you outside. When you get to the car, they will line up, and one group at a time will hand you their balloon for filling.

3. When you reach the car, tell the students that as soon as you fill their balloons, they are to work together to tie off the neck of the balloon tightly with the twist-tie. One student should hold the balloon closed, while the other student twists the neck of the balloon to prevent gas escaping and puts on the twist-tie.

4. Collect each of the samples in turn. Collect two extra samples in case a group loses its sample. Return to the classroom.

Conducting the Experiment:

1. When the students are settled into their groups, tell them that each group will be testing four gas samples. List them on the board.

a. Car exhaust (Point out that the groups already have their car exhaust samples.)

b. Air from this room

c. Carbon dioxide from vinegar and baking soda

d. Human breath

2. To collect a sample of air from the room, use an air pump to blow up a second balloon (of a different color) to approximately four inches in diameter. Groups can share the pump. Remind the students to work together in pairs while doing the tests, and for everyone to take turns using the equipment.

3. As students do steps 2, 3, and 4 below, they should list the balloon color and gas samples on a separate sheet of paper. Later this information can be transferred to their data sheets that are handed out in step 7.

4. Explain why there are five cups-relating that four of them are the four gas samples and one of them is the control.

5. Make and collect pure carbon dioxide as follows:

a. Pour 3½ oz. (100 ml) of vinegar into a large bottle.

Measure four level tablespoons of baking soda into a separate container, then add to vinegar. List these amounts on the chalkboard.

b. The carbon dioxide will drive out all of the air in the bottle in less than a second. Quickly stretch the neck of a balloon of a different color over the opening to catch the escaping gas. The balloon should inflate to about four inches in diameter.

c. Tightly tie off the neck of the balloon with a twist-tie. Helpful hint: twist the balloon neck immediately after collecting the gas so no gas escapes before you get a chance to tie it off.

6. Ask the class how they think they should collect the human breath sample? (Answer: By blowing up a balloon. Make sure it is a different color from the color chosen for the other gas samples.)

7. Tell the students that after they have collected all four gas samples, they should make them the same size by adjusting each balloon so it just passes through the hole in a large roll of tape.

8. Hand out the data sheets and summarize the procedures.

9. Tell the students to record on their data sheets which gas sample will be put into which color balloon.

a. Pour 1/2 oz. (15 ml) of BTB into each of five small plastic cups.

b. Insert a plastic straw into the balloon’s neck, and secure it with a second twist-tie.

c. With one person holding the straw in the cup, the other person carefully releases pressure on the balloon’s neck by undoing the first twist-tie, so all the gas slowly bubbles through the BTB solution.

10. After they do each test, they should record the color of the BTB in the space at the bottom.

11. Tell the groups to keep the BTB solution in the cups until the end of the experiment, so they can compare the colors. When they finish, they should fill in the names of the four samples along the line on their data sheets, from least to most carbon dioxide. Refer to the scale you have drawn on the chalkboard.

Helpful Hints During the Activity:

1. During the activity, educators should circulate among the groups to help as needed. Watch to see that no baking soda gets into any of the BTB tests (since baking soda is a base, it will neutralize carbonic acid, and prevent the BTB from changing to yellow/green). Also make sure students are viewing their BTB samples against the white paper of the observation sheet before they determine the color.

2. Remind students to fill in their results on the “color scale” on their data sheet, which indicates the relative amounts of CO2 in the samples, before they discard any of their BTB test solutions.

3. As the students finish, tell them to fill in the final section of the data sheet, which asks them to express the findings in writing.

Discussing the Results:

1. When all groups have finished, have the class put their equipment (except for the data sheets) on the trays, and put the trays aside.

2. Ask the class which gas sample had the highest carbon dioxide content. Using the scale on the chalkboard, record the results. Fill in names of the other samples the same way. When there are conflicting results, list each gas sample and write the number of groups that had each result.

3. Ask the students to look at the data from all the lab groups and to say what they think the data tell us about the amount of carbon dioxide in equal volume samples of gas from these sources. (Most groups find that the order, from least to most carbon dioxide, is: air, human breath, auto exhaust, and almost pure carbon dioxide from the baking soda and vinegar reaction.)

4. Ask the class if the results confirmed their expectations or if they were surprised at the outcome.

5. Ask the students what additional information they would need to determine the relative amounts of carbon dioxide contributed to the atmosphere by humans breathing, and by humans driving cars. (Answer: They would need to know the number of humans versus the number of cars, how much time cars are driven on average, the volume per minute of gas exhaled by humans and “exhausted” by cars.)

6. Ask the class to suggest which of the sources of carbon dioxide they have measured (car exhaust and human breath) could be reduced. How might this be done?

7. Remind the students that some carbon dioxide is needed in the atmosphere, or Earth would freeze. The problem is one of balance. Many researchers think too much carbon dioxide is being added to the atmosphere from burning fossil fuels such as gasoline, natural gas, coal, and oil. These fuels are formed by the decay of plants and animals that lived long ago. Each year people burn fossil fuels that required about 1 million years to form. There is no quick way to recapture the carbon dioxide from the atmosphere and to replace those fuels. Scientists who have measured the amount of carbon dioxide from different sources found carbon dioxide released by the United States comes from the following sources:

Residential 21.0%

Commercial 17.7%

Industrial 28.8%

Transportation 32.4%

Emissions from electricity generators, which provide electricity to the end-use sectors listed above, are allocated proportionately to the electricity consumed in each sector. Taken as a whole, electricity generator emissions count for 39.4 percent of the total US energy-related carbon dioxide emissions.

The consumption of energy in the form of fossil fuel combustion is the largest single contributor to greenhouse gas emissions in the United States and the world. The largest single source of emissions is, by far, motor gasoline, which generated 1,143.7 million metric tons of carbon dioxide in 2003.

8. Hand out the follow-up assignment,
Carbon Dioxide In the Atmosphere: Who Contributes and How Much?
Discuss the bar graphs to make sure the students understand what they represent. For example, ask for a volunteer to tell you:

“What is the current population of China?”

“What percentage of the world’s population lives in the United States?”

“What percentage of the world’s carbon dioxide is produced by Russia?”

Ask the students to complete the worksheet and give them a due date.

ASSESSMENT

The completed data sheets (especially the conclusions) and the written answers to the follow-up assignment worksheet can serve as assessments.

Needs Improvement—Data are incorrect on two or more gas samples; conclusions are vague or incorrect.

Satisfactory—Data correct on gas samples; most or all worksheet answers are correct; conclusions on both worksheets are general and correct.

Excellent—Data correct on all gas samples; worksheet answers are correct; conclusions correct and described in detail.

EXTENSION

Here are some questions for further investigation that have particular relevance to global warming issues. Any or all of these questions could be investigated by students, by collecting a sample of gas in a balloon from each source, adjusting the balloon to a standard size, and testing it with BTB, as described in this session.

Do older cars produce a higher concentration of carbon dioxide in their exhaust than newer cars?

Do cars with pollution control equipment produce less carbon dioxide than cars that do not have such equipment?

Do diesel engines produce more or less carbon dioxide than gasoline engines?

The reasoning used and conclusions reached in the following activities could provide the basis for an excellent discussion of the significance of these estimates when considering possible solutions.

Have students use a plastic garbage bag to collect a volume of gas exhausted by a car in a certain time period. Have them record the time it takes to fill the bag with that volume, estimate the volume of gas collected, and the amount of time an average car would run each day. From that data, the total estimated amount of exhaust gas that a car generates in a year can be calculated.

Students could go on to estimate the volume of human exhaust (breath) exhaled by one person in a year. Ask them to figure out how they might go about estimating this.

Finally, students could compare their estimates of the volume of exhaust contributed by a car in one year to that contributed by one person in one year.

When cleaning bottles and cups, simply rinse with water. Do NOT use soap! Soap residue will bubble and will affect the pH of the gas test. If soap has been used inadvertently, make sure the bottles are extremely well rinsed with water.

USING BTB

Bromothymol blue solution (BTB), used here to test for the presence and relative concentration of carbon dioxide, is an acid/base indicator that measures the pH range from 6.0 (yellow) through green to 7.6 (blue). The blue BTB solution moves toward the yellow (or more acidic) end of the indicator range with increasing concentrations of CO2 due to the carbonic acid created when the CO2 is bubbled through water.

BTB is a much-used chemical indicator and is not considered dangerous given normal precautions and conditions. However, BTB should be handled using standard lab safety procedures. When preparing the solution, protective gloves and goggles are recommended. While there are little data available on adverse health effects, spills of BTB on bare skin may be irritating, and the affected area should be washed immediately with soap or mild detergent and large amounts of water. If the eyes are affected, they should also be washed out with large amounts of water. If BTB is swallowed, it may cause gastrointestinal irritation. Do not induce vomiting; get medical attention immediately.

ACIDS AND BASES

The simplest atom is hydrogen. Its nucleus is a proton, which has a positive electrical charge. Around the nucleus is an electron, which has a negative electrical charge. When a hydrogen atom loses its electron, all that is left is the proton. A proton is symbolized as H+ since it is the nucleus of a hydrogen atom and has a net positive charge.

Chemicals made up of molecules from which protons (H+) may easily detach are called acids. The protons attach themselves to other molecules floating in the water. These positively charged particles give the acid certain chemical properties. For example, when mixed with bromothymol blue (BTB), acids turn yellow.

Other chemicals, called bases, are composed of molecules from which pairs of hydrogen and oxygen atoms easily detach. Each of these pairs of atoms has a negative electrical charge, symbolized as OH-.

When acids and bases are mixed in quantities that provide equal amounts of charged particles, this process is called neutralization.

Some educators suggest a good way to share class information at the end of this activity would be to prepare five clear cups, labeled as: Control, Exhaust, Breath, Air, CO2. After students have filled in their data sheets, the teacher could circulate around the class, having students combine their individual samples into the appropriate cup. The colors in the cups would provide a color-based class average of the results. The clear cups would allow the colors to be seen on an overhead projector display as the class discusses the results.

More refined analyses of car exhausts show that these gases contain a high proportion of CO2. In addition, there are other gases, including oxides of nitrogen (NOx) that react with water to form acids. These gases will contribute somewhat to the change in color of BTB from blue to yellow. Nitrous oxides are also greenhouse gases.

CREDIT

“Sources of Carbon Dioxide in the Atmosphere” reprinted from the Great Explorations in Math and Science (GEMS) teacher’s guide entitled Global Warming and the Greenhouse Effect, copyright by The Regents of the University of California, and used with permission. For more information, visit the Web site at:
www.lhsgems.org

Note to Teachers: This lesson and others relating to National Geographic’s Strange Days on Planet Earth can be found online at
www.pbs.org/strangedays/